The present invention relates to a method of manufacturing pneumatic tires, and, more specifically, to a method of improving tire uniformity during the tire manufacturing process.
A typical pneumatic radial tire includes a tread, a belt structure and a carcass. The carcass has a pair of inextensible beads, one or more plies extending between and wrapping around the beads, two sidewalls, and an apex (rubber filler) over each bead.
In the tire manufacturing process, a green tire carcass (xe2x80x9cgreenxe2x80x9d meaning as yet uncured and still tacky) is built by wrapping a length of green innerliner and at least one radial ply over a xe2x80x9cfirst stage building drumxe2x80x9d (or xe2x80x9cbuilding mandrelxe2x80x9d) and splicing the innerliner and ply ends together to form a cylindrical shape around the building drum. Two beads (each comprising a cable of steel filaments encased in green rubber) are then positioned over the ply, one near each side, and the ply is expanded radially outward to tighten against the beads. The portions of the plies that extend outward, beyond the beads are then turned up (wrapped around) the beads, forming xe2x80x9cturnupsxe2x80x9d. Then, green (uncured) sidewalls are applied around each side of the plies.
There are two methods of incorporating the apexes during the tire-making process. In the xe2x80x9cpre-apexingxe2x80x9d method, an apexing machine adheres (pre-assembles) an annular cross-sectional apex to each annular bead before the bead is positioned over the ply on the building drum. In the xe2x80x9cflat application of apexxe2x80x9d method, each apex is circumferentially laid down flat, as a triangular strip of green rubber (xe2x80x9cgum stripxe2x80x9d), onto the ply, next to a bead, so that one of the apex""s tacky sides adheres to the tacky green ply.
The resulting assembly, called a green tire xe2x80x9ccarcassxe2x80x9d, typically comprising innerliner, ply, beads, apexes, sidewalls, and other components such as chippers and flippers, is removed from the first stage building drum and mounted on a xe2x80x9csecond stage machinexe2x80x9d where it is inflated (xe2x80x9cblown upxe2x80x9d) to a toroidal shape, and the radially-outer surface of the carcass is pressed against a green (uncured) tread and a belt package to form a xe2x80x9cgreen tirexe2x80x9d. Then, the green tire is stitched (rolled with a roller) to remove air pockets and adhere internal surfaces together.
The resulting green tire is then mounted in a curing (vulcanization) mold, where a bladder is blown up within the tire cavity to press the tire""s outer surface tightly against the mold""s inner wails while the tire is vulcanized. In the mold, the tire""s green rubber initially softens under heat but eventually cures (stiffens through polymerization) enough to be removed from the mold and allowed to cool outside the mold where the curing reaction continues until the tire is cool. In some cases, the tire is kept inflated on a xe2x80x9cpost-cure inflation standxe2x80x9d while cooling, to keep the tire shape uniform and the ply uniformly stretched, to prevent the ply from shrinking nonuniformly when the tire is still hot from the mold.
After a tire is cured, it is typically tested for uniformity on a force variation machine (also called xe2x80x9ctire uniformity machinexe2x80x9d, xe2x80x9ctire uniformity inspecting machinexe2x80x9d and xe2x80x9ctire uniformity apparatusxe2x80x9d). The patent literature is rich with creative designs for force variation machines and their components. Almost all force variation machines share the same general principle as follows:
The tire is mounted on a rotatable test rim. The tire is inflated and pressed against a drum (called xe2x80x9cload-wheelxe2x80x9d, xe2x80x9cload rollxe2x80x9d, xe2x80x9cloading drumxe2x80x9d, or xe2x80x9ctest drumxe2x80x9d) whose axis is parallel with the tire axis. As the tire rotates against the drum, force sensors connected to the drum""s shaft measure changes in force (of the tire against the drum) in various directions (radial, axial and tangential to the tire-drum interface) as the tire rotates.
Graphs of force (from the sensors) vs. tire rotational angle are mathematically processed to yield different uniformity characteristics, each uniformity characteristic defining a different type of tire uniformity. Some uniformity characteristics are radial runout, radial force variation, axial force variation, tangential force variation, wobble, and conicity. These terms are defined in the Definition Section below.
Tire nonuniformity emanates from numerous factors in the tire making process. They are listed below in order of their occurrence in tire building sequence:
a) Deformation While On Rolls: The raw tire components (tread, sidewall innerliner, plies, beads and belts) either are rubber or have a rubber matrix and are stored on long rolls in the deformable green state. So, the tire components may be not remain uniformly thick during storage.
b) Nonuniform Placement On Building Drum: The ply cords may not be laid around the building drum with equal straightness and tension, and the two beads may not be positioned perfectly parallel (relative to each other) over the ply on the building drum.
c) Nonuniform Placement On Second Stage Machine: On the second stage machine, if the belt and tread are not positioned symmetrically over the green carcass, the green tire, and hence cured tire, will not be uniform. Later, as the green rubber is blown up, the bead and ply positions can shift nonuniformly.
d) Components Shift In The Green Tire State: Before curing, the beads and plies are held in place only by their green rubber matrix and the surrounding green rubber. As the green tire is handled, the bead and ply positions can shift nonuniformly.
e) Nonuniform Mounting In The Mold: If the green tire is not positioned symmetrically within the mold, the finished tire will not be uniform.
f) Ply Splice: The ply is stiffer at its splice (where it is doubled over itself) than at other locations causing irregularities in construction.
g) Ply Stretching and Shrinkage: In the mold, the inflated bladder tensions (stretches) the ply outward, and the ply cords, if constructed of nylon or polyester fibers, can shrink when heated, thus tensioning the ply further. Under tension, the ply slips around the bead, possibly to a different extent at different locations around the bead, and a splice slips around the bead least.
h) Nonuniform Curing: The rubber can xe2x80x9clock upxe2x80x9d (stiffen under cure) around the ply at different times at different locations, thus causing nonuniform ply stress.
Grinding Selected Locations
In the patent literature, the most commonly addressed method of correcting a uniformity characteristic is grinding off rubber from selected locations around the tread circumference. Numerous patents disclose a wide variety of methods, differing in how the grinder is interfaced with the force variation machine, how the grinding location (angular position) is determined based on force variation results, and which part of the tread (shoulder, crown, etc.) to grind. Disadvantages of grinding are that it contributes to environmental waste, reduces tread life, and leaves an unattractive surface finish.
Rotating Uninflated
U.S. Pat. No. 5,853,648 discloses rotating a tire in a vertical position, uninflated, while cooling after vulcanization.
Heating Selected Portions
U.S. Pat. Nos. 3,632,701; 3,865,527; 3,872,208 and 3,945,277 disclose various methods of reducing nonuniformities and/or flat spots of a cured uninflated tire based on heating only selected portions of the tire.
Post-inflation
Various patented methods of improving uniformity of a cured tire are based on xe2x80x9cpost-inflationxe2x80x9d, i.e., U.S. Pat. Nos. 2,963,737 and 4,420,453), defined as mounting the hot cured tire on a rim and keeping it inflated as it cools. The patented methods differ as to the inflation pressure and when to start and end the post-inflation.
Rotatingly Pressing Around Tire""s Entire Circumference when Hot
U.S. Pat. Nos. 3,529,048; 3,464,264; 3,635,610; and 3,389,193 disclose various methods to improve uniformity characteristics, all based on rotating a cured tire while pressing it against a roller, to xe2x80x9crun inxe2x80x9d, xe2x80x9ckneadxe2x80x9d, and/or xe2x80x9cbucklexe2x80x9d the tire""s surface around its entire circumference, to alleviate nonuniform stresses. The patents differ as to whether this is done when the tire is still hot from the mold, reheated, or made hot by flexural heating. They also differ as to whether this is done while the tire is inflated or uninflated.
Ionizing Radiation
U.S. Pat. No. 3,838,142 discloses correcting radial force variation by irradiating sections of the tread and/or sidewalls with ionizing radiation of high energy electrons.
Material Addition
U.S. Pat. No. 3,725,163 discloses reducing force variations by applying a small amount of adhering material to selected locations of the tread.
Shims
U.S. Pat. No. 5,060,510 discloses correcting radial force variation by inserting circular ring wedges of circumferentially-variable thickness (serving as shims) between the rim and the tire""s bead area.
Stretch Ply Cords
U.S. Pat. No. 5,365,781 (and its divisions U.S. Pat. Nos. 5,616,859 and 5,4581,176), disclose a method and apparatus to correct uniformity characteristics in a cured radial tire by permanently lengthening a portion of a carcass reinforcing member (i.e. selected ply cords) as a function of the magnitude of the uniformity characteristic. A significantly high inflation pressure permanently stretches the portion of the carcass reinforcing member: above its elastic limit and permanently lengthens it to an extent that is inversely related to a restraint (applied by a restraint ring) at that location. An alternative embodiment corrects force variation by mechanically (instead of by inflation pressure) stretching the portion of the carcass reinforcing member outwardly beyond its elastic limit. Disadvantages of this method are that the excessive stretching and permanent deformation may reduce the ply""s strength, fatigue resistance, and adhesion to the rubber, and may excessively stress the beads.
Post Cure Inflation with Variable Rim Width or Inflation Pressure
European Patent No. 888,872 discloses measuring a waveform of radial runout before vulcanization. In a first embodiment, immediately after vulcanization, the tire is post cure inflated (to elongate the ply cords) at a high temperature on a rim whose rim width is minimum at a location corresponding to the wave form peak. In a second embodiment, immediately after vulcanization, the tire is post cure inflated (to elongate the ply cords) at a high temperature, while restraining jigs are disposed against the tire shoulders.
All of the aforementioned prior art methods entail improving tire uniformity (or correcting a uniformity characteristic) after curing. It is undesirable to manufacture a tire that is nonuniform, only to correct it later, especially after it has cooled down. It is more desirable for a tire to already be uniform as it exits the mold.
The following prior art methods improve or correct uniformity before curing.
a) Adjust Belt Position over Green Carcass
U.S. Pat. No. 3,926,704 discloses measuring conicity of an unvulcaized (green) tire and adjusting the location of the belts accordingly while on a tire building machine.
b) Adjust Tire Shaping when Green
U.S. Pat. No. 5,882,452 discloses measuring vertical deviation of a green tire from circularity while clamped on a green tire building drum, and then shaping the green tire into a complete circle according to the measured value.
c) Reduce Bead Spacing and Post-cure Inflate
U.S. Pat. No. 3,039,839 discloses a method of solving tire shrinkage and distortion caused by shrinkage of nylon ply cords. The bead set (bead spacing when on the building drum) is narrowed (relative to the prior art) to increase the molding stretch during molding and impart a tire molding stretch to the cords. After molding, the tire is promptly mounted on an inflating rim (posture inflation stand) and kept inflated until it cools to below the nylon cord""s shrinkage temperature.
Disadvantages of Pre-cure Methods
The first two pre-cure methods (of ""704 and ""452) have the disadvantages of requiring a uniformity measurement procedure and requiring a corrective procedure that is specific for each tire. The third pre-cure method (of ""839) has the disadvantage of imposing a relatively high ply stretch that improves tire uniformity at the expense of other tire characteristics.
PCT/US99/24283 entitled IMPROVEMENTS IN BEAD CONSTRUCTION and related PCT/US99/24449 entitled AN IMPROVED PROCESS FOR MANUFACTURING TIRES both by Van Hoose and both having a common assignee with the present invention disclose an improved tire manufacturing process incorporating a tire having a cross-sectionally circular bead member comprising a rubber filler annulus and a bead wire annulus, surrounded by a thermoplastic cover. A tire manufactured with this process has reduced flow cracks and internal stresses.
U.S. Pat. No. 5,309,971 discloses a bead assembly having a rubber bead filler 84 (apex) and a reinforced rubberized ply flipper 86 that partially envelopes the bead and part of the bead filler. U.S. Pat. No. 4,934,431 (see FIG. 3) discloses a bead filler 6 (apex) disposed above a ring of bead wire 3 (bead) and a xe2x80x9cbead reinforcing layer 8 disposed within the folded portion of the carcass 4 and extending about the bead wire 3xe2x80x9d wherein the reinforcing layer 8 may be composed of cords of carbon fiber with a rubber latex adhesive. U.S. Pat. No. 3,163,683 discloses a tire bead assembly having a bead core 12, an apex 14 and a gum tie strip 16 of uncured rubber.
The invention relates to a method of manufacturing a pneumatic tire with improved tire uniformity. The tire has a pair of spaced beads and at least one carcass ply, extending between the beads and consisting of reinforcement cords embedded in a rubber matrix. The method is characterized by the steps of: a) forming the rubber matrix from a matrix material (such as a thermoplastic) that can be rendered plastic and rendered non-plastic, which respectively permits and restricts reorientation of one or more of the reinforcement cords relative to other materials or components of the tire, b) rendering the matrix material plastic (such as by heating the thermoplastic above its deflection temperature) before the tire begins to cure to permit the one or more of the reinforcement cords to be unrestricted and free to reorient themselves, and c) curing the tire in a tire mold while the matrix material remains plastic so that the reinforcement cords remain unrestricted and free to reorient themselves while curing within the mold so that the reinforcement cords maintain or attain a uniform tension. There can be a later step of: d) rendering the matrix material nonplastic (such as by cooling to below the deflection temperature) to restrict further reorientation of the one or more reinforcement cords subsequent to removing the tire from the tire mold.
If the matrix material is a thermoplastic, its deflection temperature is preferably above 30 degrees C., and more preferably between 121-190 degrees C. Heating of the thermoplastic can occur while within the mold. The thermoplastic can be sulfur vulcanizable, semi-sulfur vulcanizable or non sulfur vulcanizable thermoplastics. The reinforcement cords can be aramid (e.g., Kevlar(trademark)), steel, rayon, polyester or nylon.
Optionally, a portion of the bead portions can be comprised of the matrix material. For example, the thermoplastic material can be disposed between a bead and an adjacent portion of the carcass ply. Rendering the thermoplastic material plastic can permit the one or more reinforcement cords to slip with respect to a component of the tire, such as the beads.
Another method entails improving uniformity of a pneumatic tire having a pair of beads in bead portions and at least one carcass ply having reinforcement cords extending between the beads. The method is characterized by the steps of: a) forming at least a portion of the bead portions from a material (such as a thermoplastic) that can be rendered plastic and rendered non-plastic to respectively permit and restrict reorientation of one or more reinforcement cords relative to other reinforcement cords or components of the tire before the tire has been at least partially vulcanized, b) rendering the material plastic (such as by heating the thermoplastic above its deflection temperature) before the tire begins to cure to permit the reinforcement cords to reorient themselves; and c) curing the tire is a tire mold while the material remains plastic so that one or more of the reinforcement cords can reorient themselves during the curing within the mold so that the reinforcement cords maintain or acquire a uniform tension. The reorientation can be through the reinforcement cords slipping with respect to the beads. The material (that can be rendered plastic and nonplastic) can comprise the rubber matrix of the beads.
The thermoplastic preferably has a deflection temperature between 30-190 degrees C., and can be sulfur vulcanizable, semi-sulfur vulcanizable or non sulfur vulcanizable thermoplastics. The thermoplastic material can be disposed as a layer at least partially around the beads.